Each track in this mix is carefully selected so that each track key+mode is exactly a perfect fourth interval from the previous track. The result is highly consonant transitions, causing the tracks to blend melodically, and causing a consistent feeling of progression.

More detail for anybody interested:

Let’s say track 1 is in the key “A minor phrygian” … meaning the root note is A and the scale intervals are H-W-W-W-H-W-W

The following track would then be selected as 5 semitones above (in other words, 7 semitones below) the note ‘A’, and would be selected to share the same mode “phrygian”. The result, since A+5 = D (A-7 = D), would be “D minor phrygian”, meaning the root is D and the scale intervals are still H-W-W-W-H-W-W.

This means the root notes are a perfect fourth interval apart from one another, as are each of the other 6 notes in the scale. So, not only do you have melodic interplay between root notes, but all notes on the scale.